Communication systems, such as land mobile radio and cellular communications systems, are well known. Such systems typically include a plurality of radio communication units (e.g., vehicle-mounted mobiles or portable radios in a land mobile system and radio/telephones in a cellular system), one or more repeaters (e.g. non-portable radios usually located at a fixed repeater site) and other equipment used in the processing and monitoring of communications. The repeaters are typically connected to other fixed portions of the system (i.e., the infrastructure) via wire connections, whereas the repeaters communicate with communication units and/or other repeaters within the coverage area of their respective sites via a wireless link. That is, the repeaters transmit and receive information via radio frequency (RF) communication resources, typically comprising voice and/or data resources such as, for example, narrow band frequency modulated channels, time division modulated slots, carrier frequencies, frequency pairs, etc. that support wireless communications within their respective sites.
Communication systems may be classified as circuit-switched or packet-switched, referring to the way data is communicated between endpoints. Historically, radio communication systems have used circuit-switched architectures, where each endpoint (e.g., repeater and console sites) is linked, through dedicated or on-demand circuits, to a central radio system switching point, or “central switch.” The circuits providing connectivity to the central switch require a dedicated wire for each endpoint whether or not the endpoint is participating in a particular call. More recently, communication systems are beginning to use packet-switched networks using the Internet Protocol (IP). In packet-switched networks, the data that is to be transported between endpoints (or “hosts” in IP terminology) is divided into IP packets called datagrams. The datagrams include addressing information (e.g., source and destination addresses) that enables various routers forming an IP network to route the packets to the specified destination. Packet-switched networks are considered to be more efficient than circuit-switched networks because they do not require dedicated bandwidth or connections between endpoints, but rather permit communications between multiple endpoints to proceed concurrently over shared paths or connections.
In communication systems, time division multiple access (TDMA) is a commonly used technique for multiple radio communication units to share wireless links. In TDMA, the wireless links are divided into slices of time called slots. The radio communication units take turns receiving and transmitting in the different slots. In most TDMA communication systems, all of the slots are the same length of time. However, this creates a problem when the slots are used to carry packets using IP since IP packets are of variable length. The IP packets must be split among multiple slots and reassembled in the correct order.
The Internet Protocol was designed to be used in wireline communication networks employing a combination of ethernet, fiber optic links, T1 lines, etc. Such a network has a very low rate of errors in transmission of packets across the network. For this reason, IP contains no mechanism for detection of errors and retransmission of packets received with errors. The Transport Control Protocol (TCP) is often used in conjunction with IP to provide error detection and retransmission of errors. When this is done, the network is usually referred to as a TCP/IP network. A device running TCP performs detection of errors of the IP packets only at the destination endpoint. When a destination endpoint device running TCP detects an error, the packet must be transmitted from the source to the destination endpoint.
A wireless link has a much higher error rate than the wireline links traditionally used in networks employing IP. Hence, a TCP/IP communication network containing wireless links will have a high rate of packet retransmissions. These packets will have to be transmitted from one endpoint of communication to the other rather then just over the wireless link where the error occurred. Additionally, if the IP packets are split among multiple TDMA slots when transmitted over a wireless link and only one slot contains errors, the entire packet will have to be retransmitted. Such a TCP/IP communication network will have a higher traffic rate and greater delays than an all wireline communication network.
Not all packets carried in an IP network require the same kind of performance in terms of delay and level of errors. For example, a voice call between two communication units can tolerate only small delays but may be able to tolerate a certain number of errors without noticeably effecting voice quality. However, a data transfer between two computers may require error-free transmission but delay would be unimportant. Any methods of compensating for the error prone nature of the wireless links must take into account the differing types of service required for different uses of the communication network.
Accordingly, there is a need for a packet based communication system that takes into account the error-prone nature of wireless links. Such a communication system should provide for the splitting and reassembly of packets over TDMA wireless links. Additionally, the communication network should provide for the detection of errors and retransmission of portions of packets over the wireless links when necessary while taking into account the unique needs of different types of network traffic. The present invention is directed to addressing these needs.